It has long been known, that the CO.sub.2 molding process requires sodium silicate of so-called "lower molecular ratio" as a binder, and the mold is cured by gassing with either pure or concentrated CO.sub.2, the "lower molecular ratio" is understood to denote a ratio of between about 2-2.5. Since over 25 years it has been believed that silicates having a higher molecular ratio are not suitable as a CO.sub.2 process binder because they can not provide sufficient bonding strength to the mold. Thus the silicate having M.R. 2.7 has rarely been used as a CO.sub.2 process binder.
Although the conventional CO.sub.2 cured mold which is bonded with lower M.R. silicates shows a high bonding strength, it has several essential disadvantages: -- Firstly, it has a very poor collapsibility after casting. Secondly, it requires a large consumption of curing CO.sub.2, e.g., as much as the same amount by weight as that of the binder by weight. Consequently, the cost of CO.sub.2 for the molds amounts to about 2-2.5 times that of the binder. Furthermore, the used sands from the conventional CO.sub.2 process molds can not be reused beneficially and most of the used sands have to be discarded as waste, which inevitably causes undesirable soil and water pollution problems due to the higher alkali content and the higher water solubility of the lower M.R. silicate binder in the used sands. Since the CO.sub.2 process had come into wide use, these disadvantages caused the CO.sub.2 process to be replaced by other molding processes using, e.g., organic binders.
Much effort has since been expended to overcome the poor collapsibility of CO.sub.2 process molds by admixing different kinds of break-down agents. Nevertheless there is much to be desired in admixing break-down agents because a large amount of such additives usually provides a poor surface stability and a lower strength to the molds. Furthermore, organic or carbonaceous break-down agents do not work at temperatures higher than 1000.degree. C., to which the molds for iron and steel casting are subjected. It has been proposed to dilute the CO.sub.2 with air in the conventional CO.sub.2 process in order to reduce the CO.sub.2 consumption, but it has been proved that the CO.sub.2 consumption can be reduced no more than 30% by this method compared with pure CO.sub.2 curing, when the lower M.R. silicate binder is employed as described in Table 4 of the detailed description, this is supported by the following report: K. Hara et al.: "Application of CO.sub.2 -Air mixing Gas to CO.sub.2 Process" The Journal of the Japan Foundrymen's Society, No. 4, Vol. 39, p. 64, 1967.
On the other hand, it has been suggested that a better collapsibility would be achieved by using a sodium silicate binder having a relative high M.R. of about 3. But there were no effective curing methods available in practice expect for the following two methods: -- Firstly, heating the mold up to 250.degree.-300.degree. C. for a period of more than half an hour, which leads to a very low productivity and is uneconomical. Secondly, curing with dried or hot air, which can not uniformly cure the whole body of a large mold in practice because the moisture-saturated air will form water-saturated layers next to the dried layer. Furthermore, it takes at least 10 minutes to cure the test mold piece completely and the injection of dried air blows off the sand particles around the blowing hole. Thus the dried air injection can not be employed in practice.
According to the conventional CO.sub.2 process, such a mold which was bonded with higher M.R. silicate binder could not be cured properly in the usual working process on account of the following reasons: -- A mold bonded with sodium silicate having a M.R. of e.g. 3.16, can be cured rapidly by the conventional gassing method, but the bonding strength will so rapidly be lost in the short lapse of time after curing, that one day after gassing the mold will turn practically useless for casting, the silicates having a much higher M.R. than 3.16, as shown in Table 1 below are even more useless in comparison with the present invention. Thus the sodium silicate having a M.R. of not less than 2.7 could not be used beneficially as a molding binder for the conventional CO.sub.2 process. The other alkali silicates could not be used due to the same phenomena.
Contrary to the above described prior art, it has now been found why such a rapid deterioration of the bonding strength of a once cured mold which is bonded with a higher M.R. silicate occurs. The rapid deterioration of the bonding force is based essentially on the hypersensitivity of the higher M.R. silicates toward CO.sub.2. It has further been found based on the above, that the mechanism of developing a strong bonding strength with a higher M.R. silicate does not depend on the shocking and complete gelation, especially not on being shocked to gelation by concentrated CO.sub.2 injection. If the silicate film coating of the sand particles is shocked to gelation by gassing with CO.sub.2 according to the prior art, the silicate film loses it's bonding strength quite rapidly after being cured. The lower M.R. silicate binder is not so sensitive as the higher M.R. silicate, therefore the concentrated CO.sub.2 injection can provide a strong bonding force except when too much CO.sub.2 is injected, e.g., the mold is overgassed.
Further according to the above there is provided a proper method to overcome the aforementioned problem in the prior art, i.e., wherein a dilution agent for the CO.sub.2 is introduced to the gas in such an amount that the agent may inhibit the reaction between the higher M.R. silicate binder and CO.sub.2. By diluting the CO.sub.2 exceedingly with the inhibiting agent to a CO.sub.2 concentration of not more than 20%, the mold sands which are admixed with the higher M.R. silicate can be cured properly. Furthermore, the CO.sub.2 consumption for curing the mold is reduced to about 1/20-1/2 compared with that in the prior art process.
Furthermore, many other disadvantages due to the lower M.R. silicate binder, e.g., soil or water pollution problems caused from the high alkalinity of the used waste sands and difficulties in the recovery of the sand can be either eliminated or reduced by the application of a higher M.R. silicate binder.